Pressure monitor for a lung ventilator

ABSTRACT

A pressure monitor for use with a lung ventilator. An airway pressure sensor isolates the fluid in the airway from the monitor but transmits the pressure of the airway to the monitor. This pressure is applied to a low-limit detector and to a high-limit detector, the former of which is set to actuate when the pressure in the monitor drops below a certain value and the other when the pressure exceeds a certain value. There is also a supply pressure detector which detects when the supply pressure drops below a certain desired value. All three of these detectors are connected to a pneumatically-operated warning device which may be audible, visual, or both. The entire system is pneumatic and operates from the pressures provided by the supply for the lung ventilator.

United States Patent Ollivier [451 Feb. 25, 1975 PRESSURE MONITOR FOR A LUNG VENTILATOR [75] Inventor: Louis A. ()llivier, Menlo Park, Calif.

[73] Assignee: Veriilo Corporation, Richmond,

Calif.

22 Filed: May 4,1973 [21] Appl. No.: 357,323

[52] US. Cl. 128/145.8 [51] lot. Cl A6lml6/00 5 8] Field of Search 128/1458, 145.6, 145.5, l 28/lfl5.7, lf4 2, 142.2, 142.3, 147; 116/70; 137/557 [56] References Cited UNITED STATES PATENTS 3,669,108 6/1972 Sundblom l28/l45.8 3,741,208 6/1973 Jonsson l28/l45.6

Primary Examiner-Richard A. Gaudet Assistant ExaminerG. F. Dunne Attorney, Agent, or Firm-Owen, Wickersham & Erickson [57] ABSTRACT A pressure monitor for use with a lung ventilator. An airway pressure sensor isolates the fluid in the airway from the monitor but transmits the pressure of the airway to the monitor. This pressure is applied to a lowlimit detector and to a high-limit detector, the former of which is set to actuate when. the pressure in the monitor drops below a certain value and the other when the pressure exceeds a certain value. There is also a supply pressure detector which detects when the supply pressure drops below a certain desired value. All three of these detectors are connected to a pneumatically-operated warning device which may be audible, visual, or both. The entire system is pneumatic and operates from the pressures provided by the supply for the lung ventilator.

16 Claims, 5 Drawing Figures (TO SUPPLY 11) ii I lll (TO VENTILATOR 10) PATENTEU 3,867. 934 sum 3 or (TO VENTILATOR 'IO) PATENTEDFEB25|975 I 3,867, 934 sum u 95 {a FIGS Fig.4

1 PRESSURE MONITOR FOR A LUNG VENTILATOR BACKGROUND OF THE INVENTION This invention relates to a pressure monitor for use with lung ventilators to detect critical pressures in the airway and in the supply.

The monitor of the present invention is for the purpose of detecting three different kinds of emergency conditions to which mechanical lung ventilators are subject. It can monitor patients on both controlled and assisted ventilation, and it gives a warning when any of these three following emergency conditions occur:

1. When the supply pressure to the ventilator drops below a predetermined level. This condition applies particularly to pressure-operated ventilators.

2. When the pressure in the connection to the patient airways stays below a preset level for a predetermined time period. Staying below this level indicated insufficient volume is being delivered by the ventilator to the patient; there may be a gross lead in the breathing circuit, which may be at the connection of the breathing circuit to the patient. This may also indicate, in the case of assisted ventilation, the failure of the patient to initiate an inspiration within the time period.

3. When the pressure in the connection to the patient airway rises above a preset level. Rising above this level indicates that an excessive volume is being delivered by the ventilator, or that there is an increase in the airway resistance or a decrease in the lung compliance.

It will be evident that it is necessary for the doctor or nurse to know all three of these emergency conditions quickly in order to save the patient. a

An object of the present invention is to detect these warning conditions and to give both visual and audible warning, while also differentiating between the three conditions, so that the nurse or doctor or other attendant knows exactly which condition is involved and therefore does not have to take timeto find out which one it is.

It is also an object of the invention to isolate the airway gas from the gas in the monitor, so that the components of the monitor are kept free from-the condensation of moisture in the breathing circuit.

Another feature of the invention is that it is entirely pneumatic and is powered by the same supply source that supplies the ventilator.

SUMMARY OF THE INVENTION The present invention provides a pressure monitor for a lung ventilator having a supply of breathing gas under pressure and having an airway conduit for connection to a patient. The invention comprises a number of functional units connected together to give the desired results. A pressure regulator is connected to the supply of breathing gas to the ventilator; this regulator provides the monitor with gas at a constant preset pressure. A flow control valve has an inlet connected to the regulator and an outlet for gas at a constant rate of flow for supply to a primary conduit. The pressure in the airway is sensed by a sensing means which simultaneously isolates the gas in the airway from that in the monitor. The sensing means comprises a housing and diaphragm, with one side of the diaphragm exposed to the airway gas and its pressure and the other side con nected to the primary conduitand, therefore, to the flow control valve. This side of the diaphragm is in a chamber which is provided with a bleed outlet to the atmosphere, letting the gas from the flow control valve escape to the atmosphere; the diaphragm regulates the passage of the bleed to atmosphere, so that the pressure created in'the primary conduit is substantially identical to that in the airway.

The primary conduit is connected to both a low-limit detector and a high-limit detector. The low-limit detector has the purpose of detecting when the pressure drops below a certain predetermined level in the airway, while the high-limit detector is for detecting when the airway pressure rises above a different predetermined level- Both of these detectors are provided with a housing and a diaphragm that provides a chamber having an inlet connected to the primary conduit and having an outlet. A seal mounted on the spring-urged diaphragm is urged against the outlet to close it off, except when the pneumatic pressure in the chamber is great enough to overcome the pressure exerted by the spring means.

The high-limit detector works in conjunction with a high-pressure switch means, the connection between themnormally being supplied by gas from the regulator via a small orifice so pressure builds up in a diaphragmenclosed chamber in the high-pressure switch means. On the other side of this diaphragm is a chamber with an inlet connected to the regulator but normally closed by the diaphragm; the chamber also has an outlet for connection to an alarm device. When the predetermined upper limit of airway pressure is exceeded, the outlet in the high-pressure detector is opened, and the pressure transmitted from that detector to its switch means drops to the level of the airway pressure, which is considerably below that of the regulated pressure; this drop in pressure opens the inlet in the other chamber and'causes a flow of gas from the regulator to the alarm devices.

The low-limit detector operates in conjunction with a pneumatically-operated timer which sets a time interval. This timing means has at least one and, preferably, two diaphragm assemblies which act to provide necessary time intervals and also provide for an interrupted signal, which differs from that coming from the highpressure detector. This will be explained below.

In addition, there is a supply pressure detector which also has a diaphragm facing a chamber having an inlet connected to the supply pressure, that is to the same pressure as at the inlet to the regulator. The chamber has an outlet which leads through an orifice to the warning devices. The diaphragm carries a poppet valve which keeps the outlet closed so long as the supply pressure overcomes the pressure of a spring that bears against the diaphragm. When the supply pressure drops below a predetermined level, the spring acts on the diaphragm to open the poppet valve.

Each of the threeemergency conditions is indicated in a different manner. A low-supply pressure is indicated by a sustained sound and a continuous red flag, or other continuous visual alarm. The sound may be provided by a pneumatically-actuated reed and the flag by one of the warning devices currently on the market. Excessively low airway pressure, on the contrary, is indiciate by an interrupted sound and a winking red flag. The winking of the red flag and the interruption of the sound may be quite rapid with substantially equal periods of on and off, or they may be modified to make periodic short on periods followed by longer off periods. Finally, excessively' high airway pressure is indicated by the red flag and sound which are interrupted at the rate of the breathing frequency; these warnings persist as long as the pressure is above the preset level and then drops.

Other objects and advantages of the invention will appear from the following description of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B constitute a functional diagram of a pressure monitor for use with lung ventilators embodying the principles of the invention.

FIG. 2 is a view in side elevation of the monitor of FIGS. 1A and 1B and the members which connect it to the ventilator.

FIG. 3 is a view in front elevation of the monitor of FIGS. 1A and 1B.

FIG. 4 is a view in front elevation of the monitor of FIGS. 1-3, with the front cover removed.

DESCRIPTION OF A PREFERRED EMBODIMENT The drawings show a preferred embodiment of the invention. FIGS. 1A and 1B, taken together, provide a functional diagram of the entire device and are supplemented by additional drawings in some instances where that is necessary.

The monitor 14 of FIGS. 1A and 1B operates in conjunction with a lung ventilator 10, shown as a simple box at the bottom of FIG. 1B. The ventilator is provided with a supply 11 of breathing gas at about 35 to 50 psi, and the ventilator 10 includes an airway conduit 12 which conducts the breathing gas to the patient, in this instance, through fittings 13 and 13a.

The monitor 14 (see FIGS. 2-4) has a preset pressure regulator 17 having an inlet 18 connected to the supply 11 by a conduit 16 and having an outlet 19 leading to a conduit 20. It also includes a supply pressure detector (FIG. 1A) which is connected by a conduit 69 to an inlet chamber 42 of the preset pressure regulator 17. One of the elements connected to the conduit 20 is a flow control valve 21 (FIG. 1B) which is a needle valve adjusting the flow of the gas from the regulator 17 into a primary conduit 22. The primary conduit 22 is connected (preferably via a flexible conduit 22a) to a pressure sensor 23 which senses the pressure in the airway conduit 12 and transmits that pressure to the primary conduit 22. The primary conduit 22 is also connected to a low-limit pressure detector 24 and to a high-limit pressure detector 25. The low-limit pressure detector 24 works in conjunction with a timer 26 to detect when the pressure in the airway 12 drops below a predetermined set limit for a predetermined period of time. The high-limit detector determines when the pressure in the airway l2 exceeds a different, higher predetermined limit and is not concerned with time. It operates in conjunction with a high-limit switch 27.

The supply pressure detector 15, the timer 26, and the high-limit switch 27 are all connected to warning devices, which may comprise an audible warning device 28 and a visual warning device 29.

Details of each portion of the monitor 14 will now be discussed.

The Preset Pressure Regulator 17 (FIG. 1A)

The preset pressure regulator 17 has its inlet 18 supplied by the inlet pressure to the ventilator 10, directly from the same supply 11; it is connected to the supply 11 by the conduit 16, which terminates in a tee fitting 4 54 (FIG. 2). At its outlet 19, the regulator 17 supplies a constant pressure at a preset level. For example, the supply pressure may be at a level between 35 and psi, while the outlet 19 is preferably preset at a typical value of 20 psi.

At the inlet 18 a filter 30 is preferably provided to prevent particulate contaminants from entering the pressure regulator 17 and, of course, from going beyond it. In the pressure regulator 17 a multipart housing 31 encloses and holds a diaphragm 32 which divides a chamber 33 from a chamber 34. The chamber 34 is open to the atmosphere through a port 35 and is therefore at atmospheric pressure. The diaphragm 32 is mounted on a support member of diaphragm plate 36 which moves up and down with it as part of a diaphragm assembly 32, 36. A spring 37 mounted under compression in the chamber 34 is urged against the diaphragm plate 36.

The diaphragm assembly 32, 36 is subjected on one side to the force of the compression spring 37 and to atmospheric pressure, while on the other side it is subjected to the pressure within the chamber 33, which is the outlet pressure of the regulator 17. The diaphragm 32 maintains a balance between these two forces by actuating a poppet valve 38 through a stem 39 connected to the poppet valve 38. The poppet valve 38 opens and closes a passage 40 leading from the inlet 18 (which is at the supply pressure) to the chamber 33. Thus the poppet valve is opened to increase the flow from the supply 11 to the outlet 19 and is closed at least partially to reduce that flow when the outlet pressure tends to increase. A spring 41 in a chamber 42 urges the poppet valve 38 toward a normally closed position against a seat 43 and keeps the stem 39 in contact with the diaphragm plate 36.

Thus, the conduit 20 provides the monitor 14 with a supply of gas at the regulated pressure, which, as said before, may typically be about 20 psi.

The regulator 17, like all the other elements of the monitor 14 except the conduits l6 and 22a and the sensor 23, is in a housing 44 (FIGS. 2-4).

The Supply Pressure Detector 15 (FIG. 1A)

The supply pressure detector 15 is connected to the inlet chamber 42 of the preset pressure regulator 17 by the conduit 69. It compares the supply pressurecoming directly from the supply 11 with a preset reference pressure and produces an outlet flow signal if the pressure drops below the reference level. This flow signal is sent to the two warning assemblies 28 and 29 so that both audible and visual warnings are obtained. These warnings are continuous so long as the supply pressure is below the reference level.

The supply pressure detector 15 includes a bipartite housing 45 clamping in place a diaphragm 46 which divides the interior of the housing 45 into two chambers 47 and 48. The chamber 47 is kept at atmospheric pressure by a port 49 opening to atmosphere. Inside the chamber 47 a compression spring 50 is preset to exert a desired pressure which it exerts against a plate 51 secured to the diaphragm 46. A spring seat 52 floats on the spring 50, and the spring pressure can be adjusted by means ofa screw 53 which bears on the floating seat 52.

The conduit 69 is connected to an inlet 55 which leaves by a passage 56 into the chamber 48. An exit passage 57 leads from the chamber 48 and is centered with respect to the diaphragm 46, the initial narrow passage 57 leading to a wider portion 58 having a seat 59. In the portion 58 is mounted a poppet valve 60, which is normally urged to a closed position by a spring 61. The spring 61 is retained in place by a cap 62 which is threaded into the housing 45. The cap 62 is provided with an orifice 63 which limits the flow of the gas out from the chamber 48 when there is any flow, and this orifice 63 is connected to a conduit 64. The conduit 64 is connected by a conduit 65 and an orifice 66 to the audible warning device 28; it is also connected by conduits 67 and 68 to the visual warning device 29.

Thus, the diaphragm assembly 46. 51 is subjected on one side to the force of the compression spring 50 and on the other side to the force created by the supply pressure in the chamber 48 acting on the effective area of the diaphragm 46. Normally, the poppet valve 60 is closed, partly with the aid of its own spring 61 but largely with the aid of the supply pressure in the chamber 48 acting on the diaphragm 46. However, when the supply pressure drops below a predetermined level which may be, for example, psi, then the spring urges the diaphragm 46 and diaphragm 51 downwardly to engage the stem of the poppet valve and to move the valve 60 away from its seat 59. When the valve 60 is unseated, a flow is established from whatever is then the supply pressure to the outlet orifice 63, and from there gas flows via the conduit 64 to the warning devices 28 and 29. As long as the supply pressure is above the preset value, for example 35 psi; the poppet valve 60 remains closed and the warning devices 28 and 29 are not actuated. The Airway Pressure Sensor 23 (FIG. 1B)

The airway pressure sensor 23 is inserted in the breathing circuit to transmit to the monitor 14 a pressure signal equal to the pressure of the gas in the airway 12. The sensor 23 includes 'a housing 70 to which the fittings 13 and 13a are connected; the housing 70 and a cap 71 retain in place between them a diaphragm 72.

One side of the diaphragm 72 is subjected through passages 73 to the gas pressure in the airway 12. On the other side of the diaphragm 72 is a chamber 74 having a central outletpassage 75 leading to atmosphere via a bleed orifice 76. The diaphragm 72 is preferably not spring-loaded. The chamber 74 also has an inlet passage 77 connected to the primary conduit 22. As a matter of convenience, the primary conduit may include a flexible portion 22a leading from a fitting 78 in the housing 79 for the monitor 14, since all of the parts of this device with the exception of the airway pressure sensor 23 are contained in the same housing 79.

Thus, in the sensor 23 the diaphragm 72 is subjected on one side to the pressure of the breathing circuit, that is, the pressure in the airway 12, and on the other side to the pressure in the chamber 74 which is the signal pressure created by the gas from the regulator conduit 20 flowing through the needle valve .21, which acts as a metering valve to provide a small constant flow. In the sensor, this small flow goes from the inlet passage 77 into the chamber 74 and thence out to atmosphere via the passage 75 and the orifice 76. The relative position of the diaphragm 72 to the central opening into the outlet passage 75 determines a variable restriction for this small flow and, as a result, varies the pressure in the primary conduit 22, which is created by the small flow against the diaphragm 72. The diaphragm 72 is made very flexibleit cooperates with the flow to maintain a signal pressure which is equal to the airway pressure existing on the other side of the diaphragm 72. Since the flow is'small (the flow, for example,.may have a nominal value of 200 cc. per minute), there is no significant pressure drop in the connecting flexible tubing 22a, and thus the signal pressure atthe monitor 14 in the primary conduit 22 is the same as that at the sensor The sensor 23 may be equipped with standardized fittings for inline connection to the breathing circuit and ready adaptation thereto.

Note that a significant feature of the design is the transmission of the pressure in the airway 12 to the monitor by means of the diaphragm 72 which isolates the primary conduit 22 and the functional components of the monitor 14 from any condensation that mayexist in the actual breathing circuit.

The Low-Limit Detector 24 (FIG. 1B)

The low-limit detector 24 works in conjunction with the timer 26. Its function is to release the timing pressure of the timer 26.when the pressure in the airway 12 goes above a given reference setting. The purpose of combining the timer 26 with the low-limit detector 24 is to avoid giving a warning at every expiratory phase of the breathing cycle, when the pressure in the airway 12 drops temporarily below the low limit for which the detector 24 is set. The timer 26 delays the actuation of the warning devices for the time length of a normal expiratory phase, but if the pressure in the airway 12 continuesbelow the low limit after that time, the times 26 actuates the warning devices.

The low-limit detector comprises a housing 80 and a diaphragm 81 dividing the interior of the housing into a first chamber 82 and a second chamber 83. in the second chamber 83 is an adjustable compression spring 84.

which acts on a diaphragm plate 85 that is secured to the diaphragm and forms part of the diaphragm assembly normally urging the diaphragm 81 upwardly. A handle 86 is secured to a stem 87, which issecured by a set screw 88 to a threaded member 89 on which is mounted a spring seating member 90. The spring seating member 90 is kept from rotating by a pin 91 secured to the housing 80 and extending through a slot in the seat 90, so that rotating the handle 86 operates to change the vertical position of the seat 90 and therefore the pressure on the compression spring 84. An 0- ring 92 provides frictional resistance to rotation of the stem 87 to make the setting less susceptible to accidental change. Also, a collar 93 is fastened to the stem 87 by a 'set screw 94; the collar 93 has two pins 95 which cooperate with a pin 96 mounted to the housing 80 to establish limit stops for the rotation of the stem 87.

, The first chamber-82 is provided with a light compression spring 97 much lighter than the spring 84, which exerts a force on the diaphragm 81 opposite in direction to that of the spring 84. The chamber 82 has a passage 98 connected to the primary conduit 22 and is also provided with a threaded fitting 99 having a central passage 100 and a nozzle-'likehead 101. The diaphragm assembly 81, 85 includes a seal 102 mounted on the upper end of the assembly 81, 85 for engagement with the head 101 of the member 99 to close off the passage 100. When the seal 102 is moved away from the head 101, the timing pressure from the timer 26, which is applied .to the member 99, can pass through the passage 100 to the chamber 82. A key slot 103 enables adjustment of a threaded member to get the right position relative to the diaphragm.

When the pressure in the airway 12, which is also the pressure in the primary conduit 22 and which is present in the chamber 82, is lower than the value set by the compression spring 84, the seal 102 of the diaphragm assembly 81, 85 rests against the nozzle head 101 and closes the passage 100. When the airway pressure exceeds the value set by the compression spring 84, the diaphragm assembly 81, 85 acts to move the seal 102 away from the nozzle head 101 and to open the passage 100 so that a timing pressure existing in the timer 26 can be released into the primay conduit 22 and flow via the conduit 22a to the sensor 23 and out to atmosphere through the outlet 76. The opening of the nozzle 101 occurs with a momentary snap action, because as the seal 102 moves away from the nozzle 101 and the timing pressure begins to flow into the chamber 82, it increases momentarily the pressure in the chamber 82 and thus accelerates the motion of the diaphragm assembly 81, 85 and the movement of the seal 102 away from the nozzle 101.

The pressure range of the low limit detector 24 is typically from to 40 centimeters of water. The operating point is adjusted within that range by a 270 rotation of the knob or handle 86 fastened to the stem 87. The Timer 26 (FIG. 1B)

A timing circuit is determined by a metering valve 105 and by the capacity of a chamber within the timer 26. The metering valve 105, a needle valve, has its inlet 106 connected to the conduit from the regulator 17 and its outlet 107 connected via a conduit 108 to the timer 26 and also to the passage 100.

The function of the timer is two-fold:

1. To produce at the end of a preset time period a flow signal which may be sent to the two warning assemblies 28 and 29.

2. To characterize the flow signal, when sent, as an intermittent signal. The timer offers a choice of two modes of interruption-in the first mode, the interruption is at a high frequency with the time on being of the same order as the time off. In the other mode, the interruption is at a low frequency providing a short time on and a long time off.

The timer 26 works in conjunction with the low-limit detector 24 and therefore may be called a low-limit timer, since that is the only limit that is being timed. The pressure in the timing circuit builds up to the operating point only if the passage 100 in the low-limit detector 24 is sealed, which means that the airway pressure is then lower than the pressure setting of the lowlimit detector 24.

The timer 26 has a housing 110 and two diaphragm assemblies: a first diaphragm assembly 111 and a second diaphragm assembly 112. Each of these diaphragm assemblies includes two diaphragms. Thus, the diaphragm assembly 111 has a first diaphragm 113 and a second diaphragm 114, the upper diaphragm 113 being smaller in effective area, that is, the area exposed to the gas pressure, than is the lower diaphragm 114. Similarly, the second diaphragm assembly 112 has a first diaphragm 115 which is smaller in effective area than a second diaphragm 116.

The diaphragm assembly 111 divides an interior portion of a housing into three chambers 117, 118 and 119. Into the chamber 117 opens a passage 120 connected to the conduit 20, which leads from the pressure regulator 17. The passage 120 terminates in a nozzle 121 in the chamber 117, and the diaphragm assembly 111 carries a seal 122 which is adapted to close against the nozzle 121 or to move apart from it and open it. In the chamber 119, which is outside the larger area diaphragm 114, a coil spring 123 applies a force on the diaphragm assembly 111 in the direction which tends to urge the seal 122 against the nozzle 121.

The timing pressure from the conduit 108 is applied to the chamber 118 which lies between the two diaphragms 113 and 114 via a passage 124. This timing pressure creates on the diaphragm assembly 111 a force tending to oppose the force of the coil spring 123 since the effective area of the diaphragm 114 is larger than the effective area of the diaphragm 113.

As time passes, the pressure in the chamber 118 builds up until it reaches the operating point, where it overcomes the force of the spring 123 and moves the diaphragm assembly 111 to open the nozzle 121 and apply the regulated pressure from the conduit 20 to the chamber 117. This operation has a snap action, since the force of the pressure in the chamber 118, which tends to move the diaphragm assembly 111 and the seal 122 away from the nozzle 121, is suddenly augmented, once the nozzle 121 is opened at all, by the force created by the regulated pressure from the conduit 20, which enters the chamber 117 and acts on the smaller area diaphragm 113. This pressure in the chamber'l17 is thereupon applied via a passage 125 to an orifice 126 and sent by a conduit 127 to the two warning assemblies 28 and 29. The time that it takes the timing pressure to reach the operating point is set by the opening of the valve 105.

The pressure in the first chamber 117 is also applied via a passage 128 to an inlet 129 of a metering valve 130 which may be mounted right in the housing 110. The metering valve 130 creates a small flow which is sent from its outlet 131 via a passage 132 to the chamber 119. There, the small flow gradually builds up a pressure which creates a force on the larger area diaphragm 114 acting in a direction tending to close the seal 122 against the nozzle 121. After a given time, this force, together with the force of the spring 123, exceeds the opposing forces in the chambers 117 and 118 and closes the seal 122 against the nozzle 121. This closure also occurs with a snap action, since the opposing force ofthe first chamber 117 suddenly decreases when the regulated pressure supply is shut off from it, the pressure in the chamber 117 decreasing rapidly as a result of the outlet flow to the conduit 127. When the chamber 117 is exhausted, the flow signal to the warning assemblies 28 and 29 ceases.

In the meantime (if the pressure in the airway 12 remains lower than the setting of the low-limit detector 24), the timing pressure in the chamber 118 continues to build up, and when it reaches a value which overcomes the force of the spring 123 and the force created by the pressure remaining in the chamber 119, the first diaphragm assembly 111 is moved with a snap action to open the nozzle 121 and again produce the flow signal to the warning assemblies 28 and 29. This, in turn, activates the small flow to the chamber 119 which again stops the flow signal after another given time determined by the setting of the metering valve 130. Thus, the setting of the metering valve 130 determines the frequency at which the flow signal is interrupted; the time on is comparable in duration to the time off.

Through this process, the timer 25 generates its flow signal in the conduit 127, which is interrupted at a selected frequency. The interrupted signal persists so long as the timing pressure continues to build up. It stops only when the airway pressure exceeds the pressure setting of the low-limit detector 24, and then, of course, it is shut off. This mode of interrupting the flow signal occurs at a high frequency in the order of 50-100 times per minute, and the time on is approximately of the same order as the time off.

The second diaphragm assembly 112 provides an alternate mode of interrupting the flow signal in the conduit 127, so that there will be a short time on but a time off of longer duration than what has been described heretofore. The two diaphragms 115 and 116 define three chambers 135, 136 and 137. Thechamber 135 is open to the atmosphere via a passage 138 and lies outside the smaller area diaphragm 115. There, a seal 140 is supported by the diaphragm assembly 112 and acts to open or close an orifice 141, which is supplied by the timing pressure from the conduit 108 through a passage 142. The chamber 137, which is outside the larger-area diaphragm 116 and is maintained at atmospheric pressure by a passage 139, contains a coil spring 143 which bears against the diaphragm assembly 112 in a direction tending to force the seal 140 to close against the orifice 141. The force of this spring 143 may have either of two values; which it has is determined by the amount of compression that is set by a two-position toggle lever 144 having a cam face 145 and acting against a spring seating sleeve 146. The pressure of the outlet 131 of the metering valve 130 (the same pressure as that in the chamber 119) is applied by a passage 147 to the chamber 136 which lies between the two diaphragms 115 and 116, and this pressure creates a force which opposes the force of the coil spring 143.

If the toggle lever 144 is set in its normal position, it compresses the coil spring 143 to the higher compression value. This value is such that the pressure applied to the chamber 136 cannot reach the level required to-overcome the force of the spring 143 and move the diaphragm .assembly 112 to open theorifice 141.

In a second mode of interrupting theflow signal, the toggle lever 144. is set to its suction position, which compresses the coil spring 143 to a lower value. In this event, the pressure applied to the chamber 136 is high enough to overcome the force of the coil spring 143 and to move the diaphragm assembly 112 to open the orifice 141. Opening this orifice.141 enables the timing pressure to exhaust through the chamber 135 to the atmosphere via the passage 138. This exhaust, in turn,

makes the first diaphragm assembly 111 move the seal 122 upwardly to close off the nozzle 121 and stop the flow signal in the conduit 127. Then, the pressure in the chamber 119 and also, of course, in the chamber 136 is released through the metering valve 130. The lower value of the force of the coil spring 143 is thereupon sufficient to seal the orifice 141 as the seal 140 moves against it. When this timing circuit is thus sealed, the timing pressure starts to build up again from a low-level value, and after a given time the pressure again reaches the operating point of the first diaphragm assembly 11 1 to create the flow signal. This flow signal again is interrupted after a short time by the second diaphragm assembly 112, as described.

Through this process, the timer 26 has generated a flow signal which is interrupted in such a way that the time on is short, usually less than 1 second, while the timeoff is long, usually 5 to 10 seconds. This interrupted signal persists as long as the airway pressure remains lower than the pressure setting of the low-limit chamber 152 and a second chamber 153. The second, chamber 153 includes a spring 154 which is mounted on a seat 155 and bears against a diaphragm plate 156. The position of the seat 155 and therefore the pressure of the spring 154 is determined by know 157 which is attached to a stem 158, and the stem 158 is attached by a set screw 159 to an adjusting stud 160 to which the seat 155 is threaded. The seat 155 is held against rotation by a pin 161 secured to the housing 150. A collar 162 is attached by a set screw 163 to the stem 158, and it has two pins 164 which cooperate with a pin 165 secured to the housing 150 to determine the limits within which the spring compression can be set.

The first chamber 152 is connected by a passage 166 to the primary conduit 22 and has a threaded fitting 167 with a central passage 168 terminating in a nozzle orifice 169 which is closed or opened by a seal 170 attached to the diaphragm assembly 151, 156. The passage 168 is connected to the high-limit switch 27 by a conduit 171. Thus, the operation and structure are substantially identical, so far, to thatof the low-limit detector 24, though the high-limit detector 25 is used in a different manner.

When the pressure in the airway 12v and therefore in the primary conduit 22 is lower than the values set by the compression spring 154, the seal 170 rests against the nozzle-169 and closes the passage 168. When the airway pressure exceeds the values set by the compression spring 154,v the diaphragm assembly 151, 156 moves the seal 170 away from the nozzle 169 and opens the passage 168, through which the primary conduit 22 is connected to the conduit 171, which, iri turn, is connected to the high-limit switch 27 discussed in the next section. This will result in a drop in pressure in the high-limit switch 27 because of an arrangement now to be explained. Thehousing 150'is preferably supplied with a small inlet orifice, 172 which is connected to the conduit 20 and therefore is supplied by the outlet pressure of the pressure regulator 17.'The outlet of the orifice 172 is connected by a passage 173 to the conduit 171 and therefore to the switch 27.

Thus, when the seal 170 is moved away from the nozzle 169, the pressure in the conduit 171 is released and goes down to the airway pressure level, since the passage through the nozzle 169 is much larger than the .inlet orifice 172. Release of this pressure occurs with a momentary snap action, because as the seal 170 moves away slightly from the nozzle 169, the pressure in the conduit 171 begins to release and increases momentarily the pressure in the chamber 152 above the airway pressure sent by the detector 23 and this increase in pressure in the chamber 152 accelerates the motion of the diaphragm assembly 151, 156 and the seal 170 away from the nozzle 169.

The pressure range of the high-limit detector may be from about to about 80 centimeters of water, and the operating point is adjusted within that range by a 270 rotation of the knob 157.

The High-Limit Switch 27 (FIG. 1A)

The switch 27 works in conjunction with the highlimit detector 25. As the circuit pressure controlled by the detector 25 is released to a very low value, the switch 27 generates a flow signal which is sent to the two warning assemblies 28 and 29.

In the switch 27 a housing 175 is divided by a diaphragm 176 into a sealed first chamber 177 and a second chamber 178. The sealed first chamber 177 is connected to the conduit 17], whereas the second chamher 178 has an inlet orifice 180 that is connected by a conduit 179 to the outlet line 20 from the pressure regulator 17. The chamber 178 also has an outlet 181 which is connected through an orifice 182 to a conduit 183 that joins the conduits 65 and 67 to the warning devices 28 and 29.

The circuit pressure applied to the sealed chamber 177 is normally that of the outlet pressure of the regulator 17. It builds up to this value by the passage of gas from the conduit 20 through the orifice 172. Then, when the nozzle 169 is open and the conduit 171 communicates with the interior of the chamber 152 and hence with the pressure in the primary conduit 22, the pressure in the chamber 177 suddenly drops to about the level in the airway conduit 12, which is very low in relation to that in the conduit 20. As a result, the diaphragm 176 moves away from the inlet orifice 180 and a flow signal is sent to the warning devices 28 and 29.

The two orifices 180 and 181 function as a pressure divider: the pressure created between the two orifices 180 and 181 is at an intermediate level which is below the regulated pressure level in order to keep the diaphragm 176 away from the inlet orifice 180 and maintain the flow signal. The flow signal stops when the airway pressure returns below the setting of the high-limit detector 25. When the nozzle 169 is sealed again, the circuit pressure in the sealed chamber 177 builds up to the level of the regulated pressure in the conduit 20 and forces the diaphragm 176 in the switch 27 to seal the inlet orifice 180 and shut off the flow signal.

The Audible Warning Device 28 (FIG. 1A)

The audible warning device 28 may be any of several types available generally on the market. For example, its functional part may be a vibrating reed of a harmonica type which produces a sound at a preset frequency when a flow signal is applied to it. The sound stops as soon as the flow signal is shut off. The flow signal is applied to the audible warning device via the conduit 65 which connects to the conduits 64, 67 and 183, and through the conduit 67, to the conduit 127. In the conduit 65 there is a restriction 66 which creates a small pressure such as 2 or 3 psi upstream of the restriction when there is a flow signal, this pressure being used to actuate the visual warning device 29.

The Visual Warning Device 29 (FIG. 1A)

The visual warning device 29 may be a commercially available component such as the indicator sold under the trademark Rotowink. In this device, pressure is applied to a diaphragm assembly which rotates an indicating ball, the ball being colored in such a way that it shows black when it is not actuated, bright red when it is actuated. The pressure for actuation is taken off from the flow signal line upstream of the orifice 66. The Pressur-Limit Valve Relief (FIG. 1B)

The description of the airway pressure sensor 23 indicated that a small constant flow is supplied to the line 22a connecting the sensor 23 to the monitor 14 in order to generate a signal pressure equal to the pressure in the airway 12. If the outside connecting line should be accidentally pinched or plugged, the small flow could ultimately build up in the low-limit detector 24 and in the high-limit detector 25 to a value equal to the outlet pressure of the pressure regulator 17. To prevent this contingency from happening, a pressure-limit relief valve 190 is connected to the pressure signal line 22. This valve is set to release a small flow when the pressure signal exceeds 2 psi. A seal 191 is applied via the force of an adjustable compression spring 192 against a seat 193 at which the inlet 194 of the valve 190 ends. The signal pressure is applied to the area of the seal 191 exposed by the seat 193. When the pressure creates a force exceeding that of the compression spring 192, the seal 191 is moved away from the seat 193 and the excessive signal pressure is released to atmosphere via a passage 195.

Operation of the Monitor 14 A typical operation of the monitor 14 will now be described. The sensor 23 is inserted into the breathing circuit of a ventilator 10 at the point where it is desired to monitor the airway pressure. Typically this may be the inlet of an exhalation manifold. The pressure signal is transmitted from the sensor 23 to the monitor proper by small diameter flexible tubing 22a.

The monitor is supplied by a pressure of 35-50 psi from the supply 11, and this is appliedboth to the inlet 18 of the pressure regulator 17 and to the inlet 55 of the supply pressure detector 15. The-detector 15 looks at the supply pressure and creates a flow signal in the conduit 64 and therefore in the conduits 65 and 67 if the pressure from the supply drops below a reference setting such as a nominal 35 psi. The flow signal, if it occurs, actuates the two warning assemblies 28 and 29 to give both audible and visual warning. In both instances, the signal is sustained so that the warning sound and the red flag are sustained as long as the supply pressure is below the reference setting.

A signal pressure equal to the airway pressure is generated in the line 22a and transmitted to the primary or signal conduit 22. The signal pressure is applied to both the low-limit detector 24 and the high-limit detector 25. Meanwhile, pressure is also generated in the timing circuit of the timer 26. If the airway pressure remains below the setting of the low-limit detector 24, pressure of the timing circuit will rise to the operating point of the timer 26. At that time a flow signal is created, and the cycling network built in the timer 26 characterizes the flow signal in one of the two alternate ways: (1) When the toggle lever 144 is set to its normal position, the flow signal is interrupted at a high frequency of 50100 times per minute. The warning is given by rapidly interrupted sound and a fast winking red flag with the on time and off time being approximately equal. This mode of operation is more disturbing and more likely to attract attention than the other mode of operation. (2) The other mode of operation, which is principally intended for the condition in which the breathing circuit is disconnected purposely as in a suction operation, and where it is desirable to reduce the disturbance caused by a sustained fast interruption of the warning, is obtained simply by moving the toggle lever 144 to its suction position. Then actuation results in the warning giving a sound for somewhat less than 1 second while the red flag is on about the same time and then for 5-10 seconds the warning is off. This shows that the airway pressure is indeed lower than the set value of the low limit detector 24, but is less disturbing than the other mode of operation.

When the ventilator operates normally and the breathing circuit is effectively connected to the patient, the airway pressure exceeds the setting of the low-limit detector during each inspiratory phase. This releases the pressure of the timing circuit to the airway pressure level without actuating either warning device 28 or 29. Since the time that it takes the timing pressure to reach the operating point is longer than the breathing cycle, the operating point is never reached so long as operation continues in a satisfactory condition.

In the high-limit detector 25 the airway pressure is compared to a reference setting and if it exceeds that setting, the switching circuit associated with the detector 25 creates a flow signal which actuates the warning assemblies 28 and 29. in this instance, the warning sound and the red flag persist as long as the airway pressure is above the setting of the detector 25. This, however, does not occur during the expiratory phase, and therefore the signal is interrupted in about the same pattern as the breathing cycle.

Thus, the monitor 14 gives a warning for three different alarm conditions and characterizes the warning so that each condition can be identified by the nature of the sound or red flag display, or both:

1. A sustained, uninterrupted warning indicates low supply pressure.

2..A warning with rapid interruptions indicates that the airway pressure is not building up to the desired level in the breathing circuit; in the alternate mode, the periodic short warning indicates the same thing.

3. A warning which is interrupted at the rate of the breathing frequency indicates that the airway pressure in the breathing circuit exceeds the desired level during the inspiratory phase.

When awhole series of monitors 14 is used, a red flag of a visual warning device 29 may be used to identify which monitor 14 is giving the warning. There need be only one audible warning device 28 even with a series.

To those skilled in the art to which this invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.

I claim: l. A fully pneumatic pressure monitor for a lung ventilator having a supply of breathing gas under pressure and an airway conduit for connection to a patient, including in combination:

a pressure regulator adapted to be connected to said pp y pneumatically-operated warning means,

pneumatic airway pressure sensing means adapted to be connected to a said airway conduit for sensing the pressure therein,

a low-limit detector pneumatically connected to said airway pressure sensing means and having a valve that it opens when said airway pressure is above a predetermined low limit, as it should be during the inspiratory phase of the ventilators breathing cycle,

gas flow metering means connected to said regulator for supplying gas at a predetermined flow rate,

pneumatically-operated timing means connected to said gas flow metering means and having a control valve connected to said pressure regulator and that. opens when the gas from said metering means builds up to a predetermined pressure level within said timing means, said timing means having means for sending a flow signal from said pressure regulator to said warning means when said control valve is opened,

said timing means being connected to said valve of said low-limit detector, whereby if said valve of said low-limit detector opens before said control valve is opened, the pressure buildup from said me tering means is bled off through said valve of said low-limit detector and said control valve does not open,

a high-limit detector pneumatically connected to said airway pressure sensing means and having a valve that opens when the airway pressure exceeds a predetermined high limit,

high-limit pneumatic switch means pneumatically connected to said regulator and having an open p0 sition wherein actuating gas from said regulator is sent to said warning means and having a closed position wherein no such gas under pressure is sent, and

pneumatic means connected to said regulator, to said switch means and to said valve of said high-limit detector, for normally holding said switch means closed except when the valve of said high-limit detector is open to bleed said pneumatic means.

2. The monitor of claim 1 wherein said timing means has means for interrupting its said flowsignal so that the signal causes the warning means to go on and off rapidly in alternation and with approximately equal .times on and off, so long-as the airway pressure is belowv supply pressure detector means having .an inlet,

adapted to be connected to said supply pressure and an outlet connected to said warning means,

means for closing off said outle'tfrom said inlet when said supply pressure is above a predetermined level, and

means for opening said outlet for passage of gas from said inlet to said warning means when said supply pressure drops below a predetermined level.

5. The monitor of claim 4 wherein the warning signals are differentiated as to the cause of the warning by apparatus, as follows: i

1. means in said supply detector means for causing said warning means to be actuated continuously for as long as said supply pressure is below said predetermined level,

2. means in said timing means for interrupting its said pressure signal and thereby produce an interrupted alarm while said airway pressure is below said predetermined level, and

3. means in said high-limit detector for causing its said pressure signal to provide a long on period followed by a long off period corresponding approximately to a patient breathing cycle.

6. The monitor of claim 4 wherein said supply pressure detector means comprises a diaphragm and a housing providing a chamber with the inlet thereinto being the inlet adapted to be connected to said supply pressure and the outlet therefrom the one connected to said warning means, said diaphragm actuating a normally closed poppet valve serving as part of said means for closing off and having spring means serving as part of said means for opening, urging said diaphragm to open said poppet valve when the pressure in said chamber drops below a predetermined value.

7. The pressure monitor of claim 1 having:

a flow control valve having an inlet connected to said regulator and an outlet for gas at a constant rate of flow,

conduit means connected to said outlet of said flow control valve and connected to said airway pressure sensing means, said low-limit detector, and said high-limit detector,

said airway pressure sensing means comprising a housing and a diaphragm dividing said housing into first and second chambers, said first chamber being adapted for connection to said airway conduit, said second chamber being connected to said conduit means and having an orifice connected to atmosphere, the passage from said second chamber to said orifice being determined by the position of said diaphragm relative to said orifice, whereby the pressure in said conduit means is maintained at the pressure of a said airway conduit when said first chamber ,is connected thereto, said diaphragm isolating said airway conduit from said conduit means.

8. The pressure monitor of claim 7 wherein said lowlimit detector comprises a housing and a diaphragm providing a third chamber with an inlet connected to said conduit means and a passage with a nozzle, seal means mounted to said diaphragm, and spring means urging said diaphragm to force said seal means to close against said nozzle except when the pneumatic pressure in said third chamber is great enough to overcome the pressure exerted by said spring means, said nozzle and seal means comprising said valve of said low-limit detector.

9. The pressure monitor of claim 8 wherein said pneumatically operated timing means comprises a housing and a diaphragm assembly with two diaphragms providing fourth, fifth, and sixth chambers, said first diaphragm having a smaller effective area than said second diaphragm and lying between said fourth and fifth chambers, while said second diaphragm lies between said fifth and sixth chambers, said fourth chamber having an inlet connected to said pressure regulator and having an outlet with a sized restricted passage, said diaphragm assembly carrying a seal and having spring means for urging said seal to close said inlet, said seal and said inlet constituting said control valve, orifice means connecting said fourth chamber to said sixth chamber for timed release of gas from said fourth chamber to build up pressure in said sixth chamher aiding in urging said seal to close, said fifth chamber being connected to the outlet of said gas flow metering means.

10. The pressure monitor of claim 7 wherein said high-limit detector comprises a housing and a diaphragm providing a chamber connected to said conduit means and a nozzle, seal means mounted to said diaphragm and, together with said nozzle, constituting said valve of said high-limit detector, said valve means having an inlet of said nozzle connected to said pneumatic means, spring means urging said diaphragm to force said seal means to close said nozzle except when the pneumatic pressure in its said chamber is great enough to overcome the pressure exerted by said spring means, said pneumatic means having a small orifice with its inlet connected to said regulator and its outlet connected to said nozzle inlet and acting on said high-limit switch so that when said nozzle is sealed closed, said pneumatic means reaches the outlet pressure of said regulator and holds said high-limit switch closed, and the outlet pressure of said pneumatic means drops to the conduit means pressure when said nozzle is opened.

11. The pressure monitor of claim 10 wherein said high-limit pneumatic switch means comprises a housing and a diaphragm providing two chambers, one said chamber being connected to the outlet from said small orifice and forming part of said pneumatic means, the other said chamber having an inlet with a sized restricted passage connected to said pressure regulator and an outlet also having a sized restricted passage, this said diaphragm closing off said inlet when the pressure in said one said chamber is substantially equal to that of said regulator outlet and opening said inlet for flow of gas to the outlet when the pressure in said one said chamber drops to the level of said conduit means.

12. The pressure monitor of claim 7 having a safety pressure relief valve connected to said conduit means for relieving pressure in excess of a predetermined value.

13. A pressure monitor for a lung ventilator having a supply of breathing gas under pressure and an airway conduit for connection to a patient, including in combination:

a pressure regulator having an inlet adapted for connection to said supply and having an outlet for gas at a constant preset pressure,

a flow control valve having an inlet connected to the outlet from said regulator and an outlet for gas at a constant rate of flow,

conduit means connected to said outlet of said flow control valve,

airway pressure sensing means comprising a housing and'a diaphragm dividing said housing into first and second chambers, said first chamber being adapted for connection to said airway conduit, said second chamber being connected to said conduit means and having an orifice connected to atmosphere, the passage from said second chamber to said orifice being determined by the position of said diaphragm relative to said orifice, whereby the pressure in said conduit means is maintained at the pressure of a said airway conduit when said first chamber is connected thereto, said diaphragm isolating said airway conduit from said conduit means,

a low-limit detector having a housing and a diaphragm providing a third chamber connected to said conduit means and having a passage, seal means mounted to said diaphragm, and spring means urging said diaphragm to force said seal means to close said passage to said third chamber except when the pneumatic pressure in said third chamber is great enough to overcome the pressure exerted by said spring. means,

pneumatically-operated timing means having a housing and a diaphragm assembly with two diaphragms providing fourth, fifth, and sixth chambers, a first of said two diaphragms having a smaller effective area than a second of said two diaphragms and lying between said fourth and fifth chambers, while said second diaphragm lies between said fifth and sixth chambers, said fourth chamber having an inlet connected to the outlet from said pressure regulator and having outlet means, said diaphragm assembly carrying a seal and having spring means for urging said seal to close said inlet, orifice means connecting said fourth chamber to said sixth chamber for timed release of gas from said fourth chamber into said sixth chamber to build up pressure in said sixth chamber aiding in urging said seal to close,

metering valve means having an inlet connected to said outlet from said regulator and an outlet connected to said passage to said third chamber and to said fifth chamber for supplying a metered flow of gas to said fifth chamber to build up pressure therein that causes said diaphragm assembly to unseat the seal from said inlet to said fourth chamber so that gas flows from said regulator to and through the outlet from said fourth chamber, unless said pressure in said conduit means is sufficient to open said passage to said third chamber, thereby bleeding gas from said metering valve means through said conduit means to said orifice of said airway pressure sensing means, before the pressure in said fifth chamber can build up to the amount needed to unseat the seal in said fourth chamber, high-limit detector having a housing and a diaphragm providing a seventh chamber in said housing connected to said conduit means and having a nozzle with an inlet outside said seventh chamber and an outlet inside said seventh chamber, seal means mounted to said diaphragm, and spring means urging said diaphragm to force said seal means to close said nozzle outlet except when the pneumatic pressure in said seventh chamber is great enough to overcome the pressure exerted by said spring means, said nozzle inlet then being connected through said seventh chamber and said conduit means to said orifice of said airway pressure sensing means, said high-limit detector also having a small orifice with its inlet connected to said regulator outlet and its outlet connected to the inlet of said nozzle,

18 high-limit pneumatic switch means having a housing and a diaphragm providing eighth and ninth chambers in said housing, said eighth chamber being sealed and connected to the inlet of said nozzle, so that when said nozzle outlet is sealed closed, said eighth chamber reaches the outlet pressure of said regulator and so that the pressure in said eighth chamber drops to the pressure of said conduit means when said nozzle is opened, said ninth chamber having an inlet with a sized restricted passage connected to said outlet of said pressure regulator and an outlet with a sized restricted passage, I

its said diaphragm closing off said inlet of said ninth chamber when the pressure in said eighth chamber is substantially equal to that of said regulator outlet and opening said inlet of said ninth chamber for flow of gas to the outlet of said ninth chamber when the pressure in said eighth chamber drops to that of said conduit means, and pneumatically-operated warning means connected to t the outlet of said ninth chamber and to said outlet means of said fourth chamber for actuation by pneumatic flow from either of them.

14. The pressure monitor of claim 13 wherein said timing means has a second diaphragm assembly with two diaphragms cooperating with the housing thereof to provide three additional chambers, a tenth chamber having a bleed to atmosphere and a nozzle connected to said fifth chamber, the second! diaphragm assembly having seal means for closing said nozzle and spring means urging it toward a closed position, an eleventh chamber between said diaphragms connected to said sixth chamber, and a twelfth chamber at atmospheric pressure and containing said spring means, the effective area of the diaphragm between said eleventh and twelfth chambers being greater than that between said tenth and eleventh chambers, a spring support sleeve in said twelfth chamber, and a toggle lever having two positions and in contact with said spring support sleeve, for establishing two different spring compression values acting on said second diaphragm assembly.

15. The monitor of claim 13 having supply pressure detector means with a diaphragm and a housing providing a tenth chamber with an inlet adapted to be connected to said supply pressure and an outlet, said diaphragm actuating a normally closed poppet valve between said tenth chamber and said outlet therefrom and having spring means urging said diaphragm to open said poppet valve when the pressure in said tenth chamber drops below a predetermined value, said outlet of said tenth chamber being connected to said warning means through a sized restricted passage.

16. The monitor of claim 13 having a safety pressure relief valve connected to said conduit means for relieving pressure therein in excess ofa predetermined value. =l l 

1. A fully pneumatic pressure monitor for a lung ventilator having a supply of breathing gas under pressure and an airway conduit for connection to a patient, including in combination: a pressure regulator adapted to be connected to said supply, pneumatically-operated warning means, pneumatic airway pressure sensing means adapted to be connected to a said airway conduit for sensing the pressure therein, a low-limit detector pneumatically connected to said airway pressure sensing means and having a valve that it opens when said airway pressure is above a predetermined low limit, as it should be during the inspiratory phase of the ventilator''s breathing cycle, gas flow metering means connected to said regulator for supplying gas at a predetermined flow rate, pneumatically-operated timing means connected to said gas flow metering means and having a control valve connected to said pressure regulator and that opens when the gas from said metering means builds up to a predetermined pressure level within said timing means, said timing means having means for sending a flow signal from said pressure regulator to said warning means when said control valve is opened, said timing means being connected to said valve of said lowlimit detector, whereby if said valve of said low-limit detector opens before said control valve is opened, the pressure buildup from said metering means is bled off through said valve of said low-limit detector and said control valve does not open, a high-limit detector pneumatically connected to said airway pressure sensing means and having a valve that opens when the airway pressure exceeds a predetermined high limit, high-limit pneumatic switch means pneumatically connected to said regulator and having an open position wherein actuating gas from said regulator is sent to said warning means and haviNg a closed position wherein no such gas under pressure is sent, and pneumatic means connected to said regulator, to said switch means and to said valve of said high-limit detector, for normally holding said switch means closed except when the valve of said high-limit detector is open to bleed said pneumatic means.
 2. The monitor of claim 1 wherein said timing means has means for interrupting its said flow signal so that the signal causes the warning means to go on and off rapidly in alternation and with approximately equal times on and off, so long as the airway pressure is below said predetermined low limit.
 2. means in said timing means for interrupting its said pressure signal and thereby produce an interrupted alarm while said airway pressure is below said predetermined level, and
 3. means in said high-limit detector for causing its said pressure signal to provide a long on period followed by a long off period corresponding approximately to a patient breathing cycle.
 3. The monitor of claim 1 wherein said timing means has means for interrupting its said flow signal so that the signal causes the warning means to go on for a short period then off for a much longer period in alternation so long as said airway pressure remains below said predetermined low limit.
 4. The monitor of claim 1 having: supply pressure detector means having an inlet adapted to be connected to said supply pressure and an outlet connected to said warning means, means for closing off said outlet from said inlet when said supply pressure is above a predetermined level, and means for opening said outlet for passage of gas from said inlet to said warning means when said supply pressure drops below a predetermined level.
 5. The monitor of claim 4 wherein the warning signals are differentiated as to the cause of the warning by apparatus, as follows:
 6. The monitor of claim 4 wherein said supply pressure detector means comprises a diaphragm and a housing providing a chamber with the inlet thereinto being the inlet adapted to be connected to said supply pressure and the outlet therefrom the one connected to said warning means, said diaphragm actuating a normally closed poppet valve serving as part of said means for closing off and having spring means serving as part of said means for opening, urging said diaphragm to open said poppet valve when the pressure in said chamber drops below a predetermined value.
 7. The pressure monitor of claim 1 having: a flow control valve having an inlet connected to said regulator and an outlet for gas at a constant rate of flow, conduit means connected to said outlet of said flow control valve and connected to said airway pressure sensing means, said low-limit detector, and said high-limit detector, said airway pressure sensing means comprising a housing and a diaphragm dividing said housing into first and second chambers, said first chamber being adapted for connection to said airway conduit, said second chamber being connected to said conduit means and having an orifice connected to atmosphere, the passage from said second chamber to said orifice being determined by the position of said diaphragm relative to said orifice, whereby the pressure in said conduit means is maintained at the pressure of a said airway conduit when said first chamber is connected thereto, said diaphragm isolating said airway conduit from said conduit means.
 8. The pressure monitor of claim 7 wherein said low-limit detector comprises a housing and a diaphragm providing a third chamber with an inlet connected to said conduit means and a passage with a nozzle, seal means mounted to said diaphragm, and spring means urging said diaphragm to force said seal means to close against said nozzle except when the pneumatic pressure in said third chamber is Great enough to overcome the pressure exerted by said spring means, said nozzle and seal means comprising said valve of said low-limit detector.
 9. The pressure monitor of claim 8 wherein said pneumatically operated timing means comprises a housing and a diaphragm assembly with two diaphragms providing fourth, fifth, and sixth chambers, said first diaphragm having a smaller effective area than said second diaphragm and lying between said fourth and fifth chambers, while said second diaphragm lies between said fifth and sixth chambers, said fourth chamber having an inlet connected to said pressure regulator and having an outlet with a sized restricted passage, said diaphragm assembly carrying a seal and having spring means for urging said seal to close said inlet, said seal and said inlet constituting said control valve, orifice means connecting said fourth chamber to said sixth chamber for timed release of gas from said fourth chamber to build up pressure in said sixth chamber aiding in urging said seal to close, said fifth chamber being connected to the outlet of said gas flow metering means.
 10. The pressure monitor of claim 7 wherein said high-limit detector comprises a housing and a diaphragm providing a chamber connected to said conduit means and a nozzle, seal means mounted to said diaphragm and, together with said nozzle, constituting said valve of said high-limit detector, said valve means having an inlet of said nozzle connected to said pneumatic means, spring means urging said diaphragm to force said seal means to close said nozzle except when the pneumatic pressure in its said chamber is great enough to overcome the pressure exerted by said spring means, said pneumatic means having a small orifice with its inlet connected to said regulator and its outlet connected to said nozzle inlet and acting on said high-limit switch so that when said nozzle is sealed closed, said pneumatic means reaches the outlet pressure of said regulator and holds said high-limit switch closed, and the outlet pressure of said pneumatic means drops to the conduit means pressure when said nozzle is opened.
 11. The pressure monitor of claim 10 wherein said high-limit pneumatic switch means comprises a housing and a diaphragm providing two chambers, one said chamber being connected to the outlet from said small orifice and forming part of said pneumatic means, the other said chamber having an inlet with a sized restricted passage connected to said pressure regulator and an outlet also having a sized restricted passage, this said diaphragm closing off said inlet when the pressure in said one said chamber is substantially equal to that of said regulator outlet and opening said inlet for flow of gas to the outlet when the pressure in said one said chamber drops to the level of said conduit means.
 12. The pressure monitor of claim 7 having a safety pressure relief valve connected to said conduit means for relieving pressure in excess of a predetermined value.
 13. A pressure monitor for a lung ventilator having a supply of breathing gas under pressure and an airway conduit for connection to a patient, including in combination: a pressure regulator having an inlet adapted for connection to said supply and having an outlet for gas at a constant preset pressure, a flow control valve having an inlet connected to the outlet from said regulator and an outlet for gas at a constant rate of flow, conduit means connected to said outlet of said flow control valve, airway pressure sensing means comprising a housing and a diaphragm dividing said housing into first and second chambers, said first chamber being adapted for connection to said airway conduit, said second chamber being connected to said conduit means and having an orifice connected to atmosphere, the passage from said second chamber to said orifice being determined by the position of said diaphragm relative to said orifice, whereby the pressure in said conduit means is maintained at the pressure of a said aiRway conduit when said first chamber is connected thereto, said diaphragm isolating said airway conduit from said conduit means, a low-limit detector having a housing and a diaphragm providing a third chamber connected to said conduit means and having a passage, seal means mounted to said diaphragm, and spring means urging said diaphragm to force said seal means to close said passage to said third chamber except when the pneumatic pressure in said third chamber is great enough to overcome the pressure exerted by said spring means, pneumatically-operated timing means having a housing and a diaphragm assembly with two diaphragms providing fourth, fifth, and sixth chambers, a first of said two diaphragms having a smaller effective area than a second of said two diaphragms and lying between said fourth and fifth chambers, while said second diaphragm lies between said fifth and sixth chambers, said fourth chamber having an inlet connected to the outlet from said pressure regulator and having outlet means, said diaphragm assembly carrying a seal and having spring means for urging said seal to close said inlet, orifice means connecting said fourth chamber to said sixth chamber for timed release of gas from said fourth chamber into said sixth chamber to build up pressure in said sixth chamber aiding in urging said seal to close, metering valve means having an inlet connected to said outlet from said regulator and an outlet connected to said passage to said third chamber and to said fifth chamber for supplying a metered flow of gas to said fifth chamber to build up pressure therein that causes said diaphragm assembly to unseat the seal from said inlet to said fourth chamber so that gas flows from said regulator to and through the outlet from said fourth chamber, unless said pressure in said conduit means is sufficient to open said passage to said third chamber, thereby bleeding gas from said metering valve means through said conduit means to said orifice of said airway pressure sensing means, before the pressure in said fifth chamber can build up to the amount needed to unseat the seal in said fourth chamber, a high-limit detector having a housing and a diaphragm providing a seventh chamber in said housing connected to said conduit means and having a nozzle with an inlet outside said seventh chamber and an outlet inside said seventh chamber, seal means mounted to said diaphragm, and spring means urging said diaphragm to force said seal means to close said nozzle outlet except when the pneumatic pressure in said seventh chamber is great enough to overcome the pressure exerted by said spring means, said nozzle inlet then being connected through said seventh chamber and said conduit means to said orifice of said airway pressure sensing means, said high-limit detector also having a small orifice with its inlet connected to said regulator outlet and its outlet connected to the inlet of said nozzle, high-limit pneumatic switch means having a housing and a diaphragm providing eighth and ninth chambers in said housing, said eighth chamber being sealed and connected to the inlet of said nozzle, so that when said nozzle outlet is sealed closed, said eighth chamber reaches the outlet pressure of said regulator and so that the pressure in said eighth chamber drops to the pressure of said conduit means when said nozzle is opened, said ninth chamber having an inlet with a sized restricted passage connected to said outlet of said pressure regulator and an outlet with a sized restricted passage, its said diaphragm closing off said inlet of said ninth chamber when the pressure in said eighth chamber is substantially equal to that of said regulator outlet and opening said inlet of said ninth chamber for flow of gas to the outlet of said ninth chamber when the pressure in said eighth chamber drops to that of said conduit means, and pneumatically-operated warning means connected to the outlet of said ninth chamber and to said outlet means of said fourth chamber for actuation by pnEumatic flow from either of them.
 14. The pressure monitor of claim 13 wherein said timing means has a second diaphragm assembly with two diaphragms cooperating with the housing thereof to provide three additional chambers, a tenth chamber having a bleed to atmosphere and a nozzle connected to said fifth chamber, the second diaphragm assembly having seal means for closing said nozzle and spring means urging it toward a closed position, an eleventh chamber between said diaphragms connected to said sixth chamber, and a twelfth chamber at atmospheric pressure and containing said spring means, the effective area of the diaphragm between said eleventh and twelfth chambers being greater than that between said tenth and eleventh chambers, a spring support sleeve in said twelfth chamber, and a toggle lever having two positions and in contact with said spring support sleeve, for establishing two different spring compression values acting on said second diaphragm assembly.
 15. The monitor of claim 13 having supply pressure detector means with a diaphragm and a housing providing a tenth chamber with an inlet adapted to be connected to said supply pressure and an outlet, said diaphragm actuating a normally closed poppet valve between said tenth chamber and said outlet therefrom and having spring means urging said diaphragm to open said poppet valve when the pressure in said tenth chamber drops below a predetermined value, said outlet of said tenth chamber being connected to said warning means through a sized restricted passage.
 16. The monitor of claim 13 having a safety pressure relief valve connected to said conduit means for relieving pressure therein in excess of a predetermined value. 